1. Technical Field
Exemplary embodiments of the present disclosure generally relate to a perovskite solar cell.
2. Description of the Related Art
Recently, the importance of a solar cell is ever-increasing as an alternative energy to fossil fuel and a measure against global warming. However, the cost of present solar cells as typified by a silicon-based solar cell is high and is a factor impeding widespread use.
Thus, various inexpensive solar cells are in research and development, among which a practical realization of a dye sensitization type solar cell announced by Graetzel et al. of École Polytechnique Fédérale de Lausanne is highly anticipated (disclosed in JP Patent No. 2664194; Nature, 353(1991)737; and J. Am. Chem. Soc., 115(1993)6382). The dye sensitization type solar cell includes a porous metal oxide semiconductor electrode on a transparent conductive glass substrate, a dye adsorbed on the surface of the porous metal oxide semiconductor electrode, an electrolyte having a reduction-oxidation pair, and a counter electrode. Photoelectric conversion efficiency is significantly enhanced by making porous the metal oxide semiconductor electrode, such as titanium oxide, to enlarge its surface area, and by conducting monomolecular adsorption of ruthenium complex as the dye.
Further, in the dye sensitization type solar cell announced by Graetzel et al., printing methods may be applied as manufacturing methods of an element. Thus, there is no need for expensive manufacturing equipment and manufacturing cost may be lowered. However, the dye sensitization type solar cell includes a volatile solvent and iodine. Accordingly, problems of decline in electric power generation efficiency due to degradation of iodine redox, and volatilization or leakage of the electrolytic solution are seen.
Examples of solid dye sensitization type solar cells that make up for the above-described problems are also disclosed. The following are specific examples of such solid dye sensitization type solar cells.    1) a solid dye sensitization type solar cell employing an inorganic semiconductor (disclosed in Semicond. Sci. Technol., 10(1995)1689; and Electrochemistry, 70(2002)432),    2) a solid dye sensitization type solar cell employing a low molecular weight organic hole transport material (disclosed in JP-H11-144773-A; Synthetic Metals, 89(1997)215; and Nature, 398(1998)583), and    3) a solid dye sensitization type solar cell employing a conductive polymer (disclosed in JP-2000-106223-A; and Chem. Lett., (1997)471).
The solid dye sensitization type solar cell disclosed in Semicond. Sci. Technol., 10(1995) employs copper iodide as material for a p-type semiconductor layer. The solid dye sensitization type solar cell disclosed in Semicond. Sci. Technol., 10(1995) exhibits comparatively good photoelectric conversion efficiency immediately after manufacture though after a few hours photoelectric conversion efficiency is halved due to an increase of crystal grains of copper iodide. The solid dye sensitization type solar cell disclosed in Electrochemistry, 70(2002)432 adds imidazoliniumthiocyanate to inhibit the crystalization of copper iodide though is insufficient.
The solid dye sensitization type solar cell employing the low molecular weight organic hole transport material was announced by Hagen et al. in Synthetic Metals, 89(1997)215, and was modified by Graetzel et al. in Nature, 398(1998)583. The solid dye sensitization type solar cell disclosed in JP-H11-144773-A employs a triphenylamine compound and includes forming a charge transport layer by vacuum deposition of the triphenylamine compound. As a result, the triphenylamine compound does not reach porous holes inside of a porous semiconductor. Accordingly, only low photoelectric conversion efficiency is obtained. The solid dye sensitization type solar cell disclosed in Nature, 398(1998)583 dissolves a hole transport material of a spiro type in an organic solvent, and obtains a composite body of nano titania particles and the hole transport material with spin coating. The optimal value of a nano titania particle film thickness is approximately 2 μm. This is extremely thin compared to a film thickness of 10 μm to 20 μm in a case in which an iodine electrolytic solution is employed. Due to the thickness being approximately 2 μm, the amount of dye adsorbed on titanium oxide is small. Accordingly, sufficient light absorption or sufficient carrier generation is difficult. Thus, the properties of the solid dye sensitization type solar cell disclosed in Nature, 398(1998)583 fall short of a solid dye sensitization type solar cell employing an electrolytic solution. A reason as to why the optimal value of the nano titania particle film thickness is approximately 2 μm is disclosed as being due to insufficient permeation of a hole transport material when the nano titania particle film thickness becomes too thick.
The solid dye sensitization type solar cell employing the conductive polymer was announced by Yanagida et al. of Osaka University in Chem. Lett., (1997)471 and employs polypyrrole. However, photoelectric conversion efficiency of the solid dye sensitization type solar cell employing the conductive polymer is also low. The solid dye sensitization type solar cell employing polythiophene derivative disclosed in JP-2000-106223-A employs an electrolytic polymerization method to form a charge transport layer on a porous titanium oxide electrode having adsorbed dye. However, there are problems of desorption of the dye from the titanium oxide or decomposition of the dye.
Recently, a perovskite solar cell in which a perovskite type compound absorbs light and generates electric power was announced by Miyasaka et al. of Toin University of Yokohama in J. Am. Chem. Soc., 131(2009)6050. The perovskite type compound employed in the perovskite solar cell is formed by mixing halogenated methylamine and lead halide.
The perovskite type compound exhibits strong absorption with respect to visible light. However, the perovskite type compound is unstable within a solution. Thus, when an iodine electrolytic solution is employed, solar cell properties are low. A perovskite solar cell in which photoelectric conversion efficiency was enhanced was announced in Science 338(2012)643. Enhancement was obtained by employing a low molecular weight organic hole transport material instead of the iodine electrolytic solution. However, even with the perovskite solar cell announced in Science 338(2012)643, it cannot be said to obtain photoelectric conversion efficiency that is sufficiently satisfactory. Thus, there is a demand for an even higher photoelectric conversion efficiency.
At present, with respect to the above-examined solar cells, there are no solar cells that perform satisfactorily.